1. Field of the Invention
The present invention relates to a method for forming pixel electrodes of a liquid crystal display (LCD), and more particularly, to a self-aligned method for forming the pixel electrodes of a thin-film transistor LCD (TFT-LCD).
2. Description of the Prior Art
Since the LCD has the advantages of portability, low power consumption, and low radiation, the LCD has been widely used in various portable information products, such as notebooks, personal digital assistants (PDA), and etc.
A color filter formed on an array (COA) is a new technology and has been commonly used to form a COA type TFT-LCD. In order to maintain the thickness of a liquid crystal layerin the TFT-LCD, namelya cell gap, plastic beads, glass beads, or glass fibers are normally interposed between two substrates of the TFT-LCD and used as spacers to precisely control the cell gap to a specific value to ensure the performance of the display. In the conventional TFT-LCD process, the spacers are positioned by spraying, and tend to be mal-distributed. Consequently, the contrast of the TFT-LCD is affected due to light scattering by the spacers that are present in the light transmitting regions, generating white point defects and reducing yield rates and the display performance. For this reason, photo spacers formed by a photolithographic process have been developed to replace the conventional plastic beads to control the dimensions and positions of the spacers and the uniformity of the cell gap to accurately raise the display performance. Moreover, the photo spacers positioned in the light opaque regions can be used to prevent the light leakage problem caused by the plastic beads.
Please refer to FIG. 1 to FIG. 3, which are schematic diagrams of a method for forming a COA type TFT-LCD 10. As shown in FIG. 1, a plurality of scan lines and a plurality of signal lines perpendicular to the scan lines (not shown in FIG. 1) are formed on a surface of a bottom glass substrate 12 of the TFT-LCD 10. The signal lines and the scan lines define a plurality of adjacent pixel electrode regions 14 on the bottom glass substrate 12. Each pixel electrode region 14 includes a TFT structure 16 comprised of a polysilicon layer, a top gate conductive layer, a gate dielectric layer, a channel layer, a source electrode, and a drain electrode (not shown in FIG. 1).
Then, a planarizing layer 18 is formed on the TFT structures 16, and a black photoresist layer (not shown in FIG. 1) is formed on the planarizing layer 18. Next, a photo-etching process (PEP) is performed to form a plurality of black matrix (BM) layers 20 in the black photoresist layer corresponding to the underlying TFT structures 16 respectively, so as to improve the contrast of the TFT-LCD 10, prevent the TFT structures 16 from generating the light leakage current, and shade the oblique leaking light during operation of the TFT-LCD 10. Thereafter, a red color filter layer (not shown in FIG. 1) is formed on the bottom glass substrate 12, and another PEP is performed to form a red color filter array (CFA) 22 in the red color filter layer. The red color filter layer is composed of a photoresist containing a red dye in an amount of 10 to 50 wt % (dry weight) or a photosensitive resin. Afterwards, a green CFA 24 and a blue CFA 26 are formed on the bottom glass substrate 12 by repeating the above-mentioned processes with dyes of different colors. Thus, an R/G/B CFA comprises the red CFA 22, the green CFA 24, and the blue CFA 26.
An overcoat layer 28 and a photoresist layer (not shown in FIG. 1) are formed on the R/G/B CFA, respectively, and a PEP is performed to form a plurality of contact holes (not shown in FIG. 1) in the overcoat layer 28, the R/G/B CFA 22, 24, 26, the BM layer 20, and the planarizing layer 18. Next, a transparent conductive layer 30 with low impedance, such as indium tin oxide (ITO), is deposited on the bottom glass substrate 12 and fills the contact holes to form a plurality of contact plugs 32 for connecting the transparent conductive layer 30 and the drain electrodes of the TFT strictures 16. Afterwards, another PEP is performed to remove portions of the transparent conductive layer 30, and an orientation film 34 is formed on the transparent conductive layer 30.
As shown in FIG. 2, another transparent conductive layer 38 is formed on a top glass substrate 36 of the TFT-LCD 10, and a plurality of protrusions 40 with a thickness of approximately 3 micrometers (xcexcm) are formed on the transparent conductive layer 38. Then, a plurality of spacers 42 with a thickness of approximately 5 xcexcm are formed on the protrusions 40. The spacers 42 are composed of photosensitive polyimide (PI) materials. Further, a spin-coating process is performed to form an orientation film 44 on the top glass substrate 36.
As shown in FIG. 3, the top glass substrate 36 and the bottom glass substrate 12 are positioned face to face, so that each protrusion 40 corresponds to a side of the pixel electrode regions 14, and each spacer 42 corresponds to an intersection of each signal line and each scan line. Then, a liquid crystal layer 46 is formed between the top glass substrate 36 and the bottom glass substrate 12 to complete the fabrication of the conventional TFT-LCD 10.
However, the conventional method for forming the TFT-LCD 10 has to utilize the PEP many times to form the BM layer 20, the R/G/B CFA 22, 24, 26, the transparent conductive layer 30 of the bottom glass substrate 12, the protrusions 40, and the spacers 42 of the top glass substrate 36, respectively, consequently leading to a complicated process and a raised production cost. In addition, due to a misalignment phenomenon caused by performing the PEP many times, the BM layer 20 may not shield the underlying TFT structures 16 perfectly, generating light leakage, and affecting the normal operation of the TFT-LCD 10.
It is therefore a primary objective of the claimed invention to provide a self-aligned method for forming pixel electrodes of the LCD to simply the process and reduce the production cost.
It is another object of the claimed invention to provide a method for forming a TFT-LCD, with spacers formed between two substrates of the TFT-LCD also functioning as a black matrix layer of the TFT-LCD.
According to the preferred embodiment of the claimed invention, a method for forming a self-aligned pixel electrode of a TFT-LCD is disclosed. The TFT-LCD is formed on a substrate that comprises a plurality of scan lines and a plurality of signal lines perpendicular to the scan lines. The scan lines and the signal lines define a plurality of adjacent pixel electrode regions on the substrate. First, a photosensitive material layer is formed on the substrate and portions of the photosensitive material layer inside the pixel electrode regions are removed to leave a residual photosensitive material layer on the scan lines and the signal lines. The residual photosensitive material layer is used as a spacer and a top surface area of the spacer is larger than a bottom surface area of the spacer. Then, a transparent conductive layer is formed on the substrate to cover the spacer and the pixel electrode regions. The transparent conductive layer that covers the pixel electrode regions is separated from the spacer so as to form the self-aligned pixel electrode.
In the claimed invention, the residual photosensitive material layer having an undercut profile formed on the scan lines and the signal lines functions as the spacer of the TFT-LCD. Therefore, the transparent conductive layer formed in a subsequent process is separated from the spacer. Therefore, the present invention has the advantages of simplifying the process, reducing cost, and solving the misalignment problem caused by performing the PEP many times.
These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.